Located in Tsukiji is the Tsukiji Fish Market which is the biggest wholesale fish and seafood market in the world. The market opens at 3 am for trading where over 400 different types of seafood exchange hands.
Tuna auctions take place at about 05:20 and end at 07:00-ish. Many gaijin tourists used to come to watch the auctions but due to a few lame individuals who cause trouble, tourists were banned from watching the auctions. I think they do allow tourists to go in depending on the season under supervision by security guards though.